This is a quite convenient dac/amp, , because it has the size of an usb key , and you can move it easily from one computer to an other. It's a relatively recent product, and I saw a head-fi moderator using it with a "high end" headphone.

I'm skeptical about the sound quality, from something that small, but then I saw the price : 250$ It's the AudioQuest DragonFly Asynchronous USB DAC.

Hence my question, how good could be something that small ? In comparison my xonar stx soundcard , is much more bulky, not portable, and not as expensive.

Agreed. There's no doubt in my mind that such products (at least some) are indistinguishably just as good, yet a fraction of the price. [I personally use a slightly larger Behringer UCA202 for my notebook, which adds RCA ins and outs, Toslink, and a volume pot for the headphone jack. Using such USB corded or dongle devices also puts less torque strain on my notebook's delicate USB jacks, too, so I avoid using these plug in box designs, whenever possible.]

Also, I make it a point to not support companies I believe to be snake oil peddlers, as much as possible, even if they happen to have a particular product in their line that seems reasonable, and I encourage others to act similarly.